Various types of organic light emitting devices are known in the known art, and may be used for different purposes. The organic light emitting device is classified into a top light emitting type organic light emitting device, a bottom light emitting type organic light emitting device, and a dual-sided light emitting type organic light emitting device. If the bottom light emitting type organic light emitting device is used in an active matrix display, a thin film transistor (TFT) is disposed at a top side of a light emitting source. Thus, a ratio (aperture ratio) of an effective display region is reduced.
The above-mentioned problems are serious in the case of when a sophisticated display requiring a large number of TFTs is manufactured. The bottom light emitting type organic light emitting device generally has the aperture ratio that is less than 40%. For example, the aperture ratio that is expected in a WXGA type display using TFTs for 14″ grade may be less than 20%. This small aperture ratio negatively affects the consumption of driving power and a life span of the OLED.
The top light emitting type organic light emitting device may be used to solve the above problems. A known organic light emitting device is manufactured by sequentially layering an anode, an organic material layer, and a cathode on a substrate, and the metal that has a small work function is used as the material of the cathode. In the top light emitting type organic light emitting device, since the upper electrode is transparent, the cathode is formed to have a very small thickness so that visible rays can be passed through the metal. In addition, in the top light emitting type organic light emitting device, in order to maximize the output of light emitted through the top side, a reflective layer is provided at a lower part of the anode that is the lower electrode. The above-mentioned known top light emitting type organic light emitting device that includes the upper electrode used as the cathode and the reflective layer at a lower part of the lower electrode is shown in FIG. 1.
In the above-mentioned top light emitting type organic light emitting device, the upper electrode and the reflective layer that is provided at the lower part of the lower electrode act as mirrors, thus light that is emitted from the light emitting layer of the organic light emitting device is reflected by the mirrors. Accordingly, the destructive interference and the constructive interference of light occur, causing a phenomenon where only light having a predetermined wavelength is maintained and the intensity of light having the remaining wavelength is reduced. The phenomenon is called the microcavity effect. In the top light emitting type organic light emitting device, the light emitting spectrum is shifted or the color coordinate is changed due to the above phenomenon. In connection with this, a distance between the mirrors is called an optical length.
An effort has been made to control the optical length so that the optical interference intensity approaches the peak according to the type of color of emitted light. For example, Korean Unexamined Patent Application Publication No. 10-2005-0048412 discloses a method of controlling a thickness of an anode that is a lower electrode of an organic light emitting device (FIG. 2). However, when the thickness of the lower electrode is controlled according to the type of color of emitted light by using masks, the type of which depends on the type of color of emitted light, during a typical sputtering process in the course of forming the lower electrode, there are problems in that a life span of the mask is reduced and alien substances are generated during the process. In addition, an uneven structure that is caused by a nonuniform thickness of the lower part of light emitting layer according to the type of color of emitted light may reduce the stability of the whole manufacturing process of the device. Furthermore, in order to control the optical length, the thickness of the organic material layer of the organic light emitting device may be controlled (FIG. 3). In this case, there are problems in that it is difficult to perform the process and driving voltage is increased.